This invention relates to a metal extrusion process to be used for forming a metal blank into a plate or a formed metal shape having one of a variety of cross sections.
Heretofore, in rough rolling of billets or shapes the total elongations of which are 4 - 7 times the original length, the metal has been rolled in 6 - 10 passes in one reversing mill. However, this reverse rolling system has low productivity and also the temperature drop in the rolled metal during the rolling operation is large, which factors are drawbacks of the reverse rolling system. Lately, in order to improve the productivity of rolling operations, a process for continuously rolling the rolled metal by a continuous rolling installation wherein 6 - 10 stands of rolling mills are disposed in series has been widely employed. With this process, it is possible to eliminate the drawbacks of the reverse rolling system, but a longer line is required and the installation becomes large in size, and it invites the problems of replacement of rolls and adjustments and maintenance, etc. Under the circumstances, in order to eliminate the foregoing drawbacks, special mills such as a planetary cross rolling mill or a swing forging mill have been developed.
These special mills have a complicated structure and are disadvantageous from the standpoint of maintenance in many respects, and also the cross section of the product becomes more less angular in shape which is not necessarily acceptable and moreover the surface skin shows spiral patterns. These are some of the various problems encountered, and make such special mills far from satisfactory.
An extrusion process which has been employed heretofore is one in which a hot blank which is accommodated in a blank accommodating container (hereinafter referred to simply as the container) is placed under great pressure by a pressure or force generating apparatus, and the blank is caused to pass through a die provided at the extrusion end of the container opposite the end provided with the pressurizing apparatus whereby the forming of the blank is carried out.
In this process, there is a feature that a large cross section reduction ratio can be achieved for the blank in one pass and also it can be given an extremely complicated cross sectional shape. However, the forming load is extremely great as compared with the other metal forming processes such as rolling process, and moreover there are problems involved in the life of the die and the lubricant, and moreover continuous mass production such as in a conventional rolling process is difficult to perform and efficiency is low.
In a conventional extrusion apparatus, the die is fixed to the container, and accordingly, in order to shape the metal blank by causing it to pass the opening portion of the die which is shaped to produce a particularly large cross sectional reduction ratio, an extremely large extruding force is required which is impossible from the standpoint of installation. Also the metal blank is tightly pressed into the container under high pressure, so that lubrication of the inner wall of the container and the die cannot be carried out, and as a result, wear and tear are caused on the container and die, and consequently repair or replacement has to be made frequently. Moreover, if the extrusion is carried out with a reduction, a slower extrusion speed is inevitable, which factors are drawbacks which greatly reduce the productivity.
Lately, in order to overcome the foregoing drawbacks, a process of extruding metal wherein the metal is cooled at the outlet side of the extrusion opening and the forming is carried out while applying drawing power which is more than 30% of the yield stress of the extruded metal has been invented. In that invention, two systems are included wherein for generating drawing power, a drawing apparatus is provided, and for further generating drawing power work rolls are driven instead of providing a die with rotary work rolls. However, in these systems, in the latter system, there is a defect due to the generation of overfill and a problem of generation of overfill scar on the portion not contacting the die formed by the work rolls. This overfill spoils the appearance of the product and not only lowers the value of the product and but also makes the product almost unsuitable for the market.
Also, when a blank is used as a semi-finished product the corner portion of which is a right angle and no rounding of the corner portion is provided, and rolling is repeatedly carried out on this semi-finished product to produce a product, there are cases where scars are generated on the product by the rolling operation which takes place after the repeated rollings. Accordingly, in order to prevent the generation of scars due to the rolling operations, it is desirable that the corner portion be chamfered to a certain degree or be rounded.